Catalytic processes using either homogeneous catalysts, i.e. those present in the same phase as the reactant, or heterogeneous catalysts, i.e. those present as a separate phase in the reaction medium, have played an important role in organic synthesis. Heterogeneous catalysts are insoluble; thus they can be readily separated from the reaction mixture and, generally, offer the potential for ready re-use. Despite these advantages, prior art heterogeneous catalysts are rather limited in the number and types of organic reactions in which they can be used. In addition, they are usually less selective than homogeneous catalysts which are typically soluble metal salts or metal complexes. Indeed, homogeneous catalysts are not only more selective than heterogeneous catalysts, but have been used to promote a wider variety of organic reactions. Nevertheless, difficulties can be encountered in separating the soluble, homogeneous catalyst, both the metal and the accompanying ligands, from the product. This not only presents problems with the purity of the product, but also makes the re-use of the homogeneous catalyst problematic. The potential loss of the ligand is particularly serious in enantioselective reactions where chiral ligands are usually quite expensive.
Over the past twenty-five years, attempts have been made to "heterogenize" the more versatile homogeneous catalysts, the primary aim being to maintain reaction activity and selectivity of the homogeneous species while at the same time significantly increasing the ease of separation from the reaction medium. One such approach to achieve "heterogenization" involves reacting a metal complex or salt with a solid support such as a polymer or a metal oxide which had been previously modified by the addition of phosphine or amine ligands to the surface of the support. Catalysis Reviews, 16, 17-37 (1974) and Chemical Reviews, 81, 109 (1981) are reviews of the earlier literature concerned with polymer supported complexes. Tetrahedron: Asymmetry, 6, 1109-1116 (1995), Tetrahedron Letters, 37, 3375-3378 (1996) and Chemische Berichte, 129, 815-821 (1996) are examples of recent references in this area. From a practical approach, these catalysts are not widely used since their activities are frequently lower than those of the corresponding homogeneous analogs. In addition, problems associated with polymer swelling and attendant mass transport difficulties can be encountered, as well as the finding that activity is frequently lost on attempted re-use. Some success has been reported in preparing polymer supported chiral, complexes, but the selectivity observed with the use of such "heterogenized" species has generally been lower than that obtained using the homogeneous catalyst itself.
"Catalysis by Supported Complexes", Studies in Surface Science and Catalysis, Volume 8, Elsevier Publishing Company, Amsterdam, 1981 is an extensive review of the earlier work concerned with the anchoring of metal complexes onto surface modified oxides. Journal of Catalysis, 157, 436-449 (1995) and Bulletin Societe de Chemie, France, 133, 351-357 (1996) are some more recent references. While these materials do not manifest significant swelling problems associated with the use of polymer supports, there are frequent reports of loss of activity on attempted re-use.
In rare instances, the oxide support does not have to be modified before the application of a metal complex. Journal of Molecular Catalysis, 88, 13-22 (1994) describes the interaction of Rh(OH) (CO) (PPh.sub.3).sub.2 with an alumina surface to give a supported catalyst for the hydrogenation of alkenes and benzene. This report also states that the presence of the Rh-OH entity is necessary for interaction with the surface of the alumina and that other complexes could not be attached to the oxide surface.
Another problem associated with prior art, catalysts made from metal complexes which are attached to either a modified polymer or metal oxide surface is that their preparation techniques are rather specific and are driven by the nature of the ligand to be attached. Hence, modification of the catalyst to introduce another, more selective ligand is usually an arduous and complex task, if it is one that can be accomplished at all. This circumstance has particular importance where the preparation of enantioselective catalysts are concerned since optimal enantiomeric excess is usually obtained using a specific ligand or class of ligands for a given reaction or substrate.
Journal of Catalysis, 152, 25-30 (1995) describes the preparation of chiral, supported aqueous-phase catalysts and their use in the preparation of naproxen. These heterogeneous catalysts have the same enantioselectivity as the homogeneous counterpart, but are 2 to 2.5 times less active.
Despite the current state of the art, there is a continuing need to develop stable heterogeneous catalysts which employ an active metal complex on an insoluble support, which catalysts are highly reactive and selective in organic reactions. Indeed, a particular need exists for the development of such catalysts which contain a chiral metal entity capable of promoting an enantioselective reaction. The term "chiral metal entity" is used herein to denote metal complexes which contain at least one chiral ligand.